Change Labels on Form Fields

Hello, I absolutely love the Coupon theme! It’s amazing and more than I expected and now I’m looking at customizing the the forms a bit.

I just want to change the labels on the forms from Username to Business Name, and simple changes like this. Can someone direct me on the best way to do that for this theme please? Perhaps let me know which file I need to change if that is the best way to do this.

Thank you for any help.